The present invention relates to fiber optic amplifiers. The Government has rights in this invention pursuant to Contract Number F33615-79-C-1789 awarded by the Department of the Air Force, Air Force Office of Scientific Research.
The concept of optical amplifiers, based upon the lasing capability of certain materials, particularly on a macroscopic level, is well known. Thus, for example, it is known to place a pumping light source and a single crystal neodymium-yttrium aluminum garnet (Nd:YAG) rod, several millimeters in diameter and several centimeters in length, in a tubular reflective cavity. For example, the light source and Nd:YAG rod may be located, respectively, to extend along the two foci of a cavity having an elliptical cross section. In such an arrangement, light emitted by the light source and reflected from the cavity walls will impinge upon the Nd:YAG rod. The light source is preferably selected to emit wavelengths corresponding to the absorption spectra of the Nd:YAG crystal so that the energy states of the neodymium ions of the crystal are inverted to an energy level above the upper lasing level. After inversion, an initial relaxation of then neodymium ions through phonon radiation yields anion population at the upper laser level. From the upper laser level, the ions will relax, to a lower energy level, emitting light of a wavelength which is characteristic of the Nd:YAG material. Advantageously, this lower energy level is above the ground level for the ions so that a rapid, phonon-assisted relaxation will occur between this lower energy level and the ground level, enabling a high inversion ratio to continue to exist between the upper laser level and this lower energy level, within the pumped ions.
With the population so inverted, as is well known from laster technology, the Nd:YAG will also provide fluorescence, that is, random emission of incoherent light. This spontaneous radiation takes place with a time constant equal to the average lifetime of ions in the inverted state is 230 microseconds for Nd:YAG.
If, after the neodymium ions of the Nd:YAG rod have been inverted, a light signal at the laser transition frequency is transmitted through the rod, the signal photons will trigger the transition of the neodymium ions, to the lower energy level, causing coherent emission of stimulated radiation, which will effectively add to the transmitted signal, thus amplifying this signal.
The absorption length of the Nd:YAG crystal at the pump wavelength (i.e., the length of material through which the illumination must traverse before 60% of the illumination is absorbed) is typically about 2 millimeters or more, and thus the Nd:YAG crystals used in amplifying structures have had diameters at least this large so that the crystal could absorb a substantial portion of the pumping radiation during the initial reflection from the cavity walls and passage through the crystal. If, during this initial traverse through the crystal, the pumping illumination is not absorbed, it is likely to be reflected by the cavity walls back to the light back to the light source, where it will be reabsorbed, it is likely to be reflected by the cavity walls back to the light source, where it will be reabsorbed, generating heat in the light source and reducing the overall efficiency of the amplifier.
When such large diameter Nd:YAG rods are used as amplifiers in fiber optic systems, it has been thought necessary to use optical components, such as lenses, to focus the light signal from the optical fiber into the Nd:YAG rod, and the amplified light signal from the Nd:YAG rod back into another fiber. Such optical systems require careful alignment and are susceptible to environmental changes, such as vibration, and thermal effects. Additionally, the optical components and the size of the Nd:YAG rod make the amplifying system relatively large, and thus impractical for certain applications. Furthermore, the large size of the Nd:YAG rod requires a large mount of input pump energy in order to maintain a high energy density within the rod and allow for a significant optical gain. Such large pump power requires high output pump light sources, generating substantial heat which must be dissipated, typically by liquid cooling of the cavity.
While amplifiers of this type are useful in many applications, such as some communication applications, use in a recirculating fiber optic gyroscope puts severe restrictions upon the amplification system. With such gyroscopes, optical fiber, typically a kilometer or more in length, is wound into a loop, and a light signal is recirculated within the loop, typically in both directions. Motion of the loop causes a phase difference between the counter-propagating light signals which may be used to measure gyroscope rotation. In such gyroscopes, the phase difference induced in one signal pass around the fiber is relatively small, and it is advantageous to recirculate the light signal within the loop as many times as possible to increase this phase difference.
In traversing a kilometer of optical fiber, an optical signal will typically lose 30 to 50 percent of its intensity. If an amplifier were placed in series with the loop, and were capable of amplifying the bidirectional counter-propagating light signals by 2 to 3 dB, it would permit a light signal to propagate many times within the loop.
Unfortunately, the relatively large size, high power and cooling requirements of prior art Nd:YAG rod amplifiers, as described above, make such amplifiers relatively impractical for high accuracy gyroscopes. These factors, of course, also limit the utility of such amplifiers in other applications, such as communication networks.